The invention relates to a shaft coupling which connects two shafts to transmit torque, wherein one shaft end has at least one recess and/or one projection, and the other cooperating shaft end has at least one projection and/or one recess and these are coupled.
Shaft couplings of this type are known. For example, one type is formed by a bar which projects from one shaft end, in particular the front end, and which engages in a corresponding groove in the front end of the other shaft. A positive lock of this type transmits torques from a driving shaft to a driven shaft. However, these shaft couplings operate fault free only when the two shafts are satisfactorily aligned. To achieve this, centering pins and bores have been provided on the housing parts that accommodate the shafts. However, this merely aligns the housing parts.
For example, this principle is used in motor vehicles, where vacuum pumps are aligned with the aid of centering pins on the cylinder head or on the engine block of an internal combustion engine. If faulty positioning or axial offset occurs on account of mounting errors and/or tolerances between the shaft and the housing, the shaft coupling may be very quickly damaged or destroyed. This happens because the bar of one shaft, which engages in the groove in the other shaft, jams. The drive to the pump may stall. An attempt to prevent this comprises allowing a specific clearance between the bars and the corresponding grooves. However, upon faulty positioning of the shafts, such play promotes so-called edge bearing. Impermissibly high area stresses or edge pressures occur, and these may rapidly wear or even destroy the shaft coupling. To counteract this, the load bearing surfaces of the bars and of the grooves are designed spherical. For spherical surfaces, flattening of the points of contact takes place under compressive forces. In excessively high compressive forces, this effect, known as "Hertzian stress", causes flattening and plastic deformation of the bar or groove.
Furthermore, the prior art suggests compensating for axial offset or angular misalignment of the shafts by interposing an articulated coupling. Such a coupling may comprise elastic articulated shafts, cardan joints or a generally known Oldham coupling. However, production of such couplings has proven to be costly. Furthermore, the overall length and weight of the overall arrangement are increased by a coupling placed onto the shaft ends. In addition, it is disadvantageous here that, in modern motor vehicle engines, particularly on the cam shaft, relatively high angular accelerations occur, which have the effect of extremely loading the ancillary units driven by the camshaft. As a result of the play in the coupling, the angular accelerations which act on the shafts of the ancillary units may even be increased by a factor of 2 to 4.
Prior art measures are not aimed at avoiding faulty positioning of the shafts, but merely at compensating for its occurrence. This leads to the disadvantages already discussed.